Prostate cancer (PCa) is the most common malignancy among men, and the inhibition of androgen biosynthesis and its action on its cognate receptors is critical to the therapeutic management of the disease. The biosynthesis of androgens is controlled in the central nervous system (CNS), but also directly in peripheral target tissues. In the CNS, lutenizing hormone release hormone (LHRH) acts on the pituitary to increase lutenizing hormone (LH) production which acts on the testis to increase androgen synthesis. In peripheral tissues, active androgens such as testosterone can also be synthesized directly in target tissues through the conversion of inactive precursors which are present at high levels in the circulation. Current therapies for the hormonal control of PCa counteract CNS control mechanisms through LHRH modulation and/or directly through androgen receptor (AR) antagonism at the site of action to inhibit the growth of hormone-sensitive PCa cells. Although receptor antagonists and LHRH analogues have proven of significant benefit in the treatment of PCa, the enzymes involved in the regulation of local target tissue androgen biosynthesis could also be important points of therapeutic intervention given their ability to regulate both centrally- and peripherally-controlled androgen biosynthesis.
Dihydroepiandrostenedione (DHEA) and androstenedione (AdT) are the major precursors of active sex steroid hormones and are present at high levels in the circulation through production by the adrenal gland. It has been estimated that the levels of DHEA and AdT are in 100–500 fold excess to that of testosterone, which represents a large depot of inactive precursor for conversion to active hormone within target tissues. The enzymes that mediate the last steps in the conversion of inactive to active sex steroids in peripheral target tissues are the 17β- hydroxysteroid dehydrogenases (17β-HSDs), members of the short-chain alcohol dehydrogenase family. At present, nine different human 17β-HSDs have been identified and substrate preferences characterized. This group of related enzymes displays unique patterns of adult tissue distribution and distinct substrate preferences suggesting the evolution of precise control mechanisms on the intracellular production of sex steroid hormones at the local tissue level. Given their key role in catalyzing the formation of active estradiol and testosterone in key target tissues of hormone action, the 17β-HSDs have been considered as potential molecular targets for pharmacological modulation for the treatment of breast and prostate cancers.
Although surgical castration results in a >90% decrease in serum testosterone, levels of testosterone in target tissues such as the prostate are only diminished by ˜50% indicating that significant local active hormone synthesis occurs in the absence of a gonadal source. A candidate enzyme for mediating this extragonadal production of active hormone is 17β-HSD3 which has been characterized for its catalysis of the reduction of androstenedione to testosterone (FIG. 1). 17β-HSD3 is expressed in peripheral tissues which are the site of the testosterone action in normal individuals, including at high levels in the seminal vesicles and testes, but has also been shown to be present in prostate tissue. Further, a variety of inactivating mutations have been reported in 17β-HSD3 at a high frequency in a male pseudohermaphroditism syndrome which results in decreased serum testosterone levels and the impairment of the sexual differentiation of the male internal reproductive organs, such as the seminal vesicles and prostate, suggesting that 17β-HSD3 participates in both central and peripheral testosterone synthesis.
17β-hydroxysteroid dehydrogenase 3 (17β-HSD3) is an essential enzyme in the biosynthesis of testosterone. It catalyzes the reduction of androstenedione, a weakly active androgen produced by the adrenal glands, to testosterone (FIG. 1). (Inano et al., Steroids, 48, 1–26, (1986) and Luu-The et al., J. Steroid Biochem. Mol. Biol., 55, 581–587 (1995)) 17β-HSD3 is expressed predominately in the adult testes and to a lesser extent in seminal vesicles and prostate tissue, an expression pattern consistent with an enzyme involved in both gonadal and peripheral target tissue androgen biosynthesis. 17β-HSD3 is responsible for the synthesis of about 60% of all active androgens in men. (Labrie, Mol. Cell. Endocrinol. 78, C113–C118 (1991)) The development and progression of hormone sensitive diseases, e.g., prostate cancer, is stimulated by androgens such as testosterone. Inhibition of 17β-HSD3 therefore provides a novel means to disrupt testosterone biosynthesis for the treatment of androgen-associated diseases. (Van Weerden et al., J. Steroid Biochem. Mol. Biol., 20, 903–907 (1990) and Liu et al., J. Clin. Endocrinol., 77, 1472–1478 (1993)) Current pharmacological treatments to prevent androgen action in androgen-associated diseases such as prostate cancer are centered on the combined use of luteinizing hormone releasing hormone (LHRH) analogues with androgen receptor antagonists (“anti-androgens”). (Labrie et al., Endocr.-Relat. Cancer, 3, 243–278 (1996); Gheiler et al., World J. Urol., 18, 190–193 (2000); Simard et al., J. Urol., 49, 580–586 (1997)) LHRH analogues interfere with central nervous system feedback mechanisms to suppress testosterone biosynthesis in the testes to produce chemical castration. However, it is estimated that up to 50% of testosterone levels remain within prostate tissue following chemical or surgical castration indicating the existence of alternate routes of testosterone biosynthesis independent of the testes. Anti-androgens are used to block the action of this remaining testosterone in prostate cancer cells by antagonizing hormone function at the level of receptor binding. Although the combined use of LHRH analogues with anti-androgens has shown success in the management of prostate cancer, these responses are largely restricted to advanced metastatic disease. Further, patients receiving these treatments ultimately become refractory and progress to a more aggressive, hormone-independent state for which there is no effective therapy.
Inhibitors of 17β-HSD3 have been described in the art. (Pittaway, Contraception, 27, 431 (1983); Labrie et al., WO99/46279; Maltais et al., J. Med. Chem., 45, 640–653 (2002); Guzi et al., WO03/022835).
The role of 17β-HSD3 in both centrally and peripherally controlled testosterone biosynthesis, suggests that 17β-HSD3 inhibitors may be beneficial for the treatment of hormone-sensitive prostate cancers. It would be desirable to have a method to identify or screen for agents that inhibit 17β-HSD3.